Decoder for pulse code modulation



July 11, 1950 A, J, RACK 2,514,671

yDECODER FOR PULSE CODE MoDuLA'rIoN y Filed sept. 25, 1947 FIG. F IG. 2

PCM, P41/ ses glea PcMNPuLsEs aacoosa N i n@ =5-ca co/vswvr calmer/r R5haer/Europ R4 ascenso 25 ourpur) PC M PULSES www SNPLING PULSES IN ATTORNE V DECODER Fon PULSE CODE Es PATENT OFFICE MoDoLATIoN 'Ai'os J'.Rack, Millington, N; J.; assig'nulto Ben" Telephone Laboratories,Incorporated, New York, N. Y., a corporationof Newy York y ,Appucauonseptember 23,1'947,sria1,1so.775,633

11 claims. (o1. 2501-27) This inventionrelates'; to receivers forcommunication `systems and more f particularly to decoders for use inthe receivingequipment of 'communication systemsl employing "pulse codemodulation. 4

,In `communieation systems utilizing what is iknown/as pulsecodetransmission, av speech Wave or other signal. to. be'transmitted issampled .periodically to lascertain its instantaneous amplitude which'is expressedby a pulse vcode analogousto a telegraphcode. l 4 l 4Onecode which conveniently may be employed in pulse codeltransmissioninvolves permutations of a fired Anumber of code elementseach of whichmay have any one of. several conditions or values.. An advantageousYcode of this type is the sol-called binary' code in which each of kthexe'dnumber'of codeelements may .have either .of'two values'. Oneadvantageous Way of representing these lvaluesis to'represent'one by apulse sometimes reierred'to as an ".on pulse and the i other by the'absence of. 'a pulse VSometimes referredto as an oi pulse,Alternatively, one valuemay be represented by a'positive pulse and ntheother byl afnegativefpulse.v The total numper of permutationsobtainable` wim'the binary c odevis equal, to '2 Where n is theynumberof code elements employed. i AfBecause fthe total number of differentramplitudes which vmay.` be represented by such a code ora xed numberofjelements is limited, it is found desirable yto .divide `the"continuous .range 'ofA amplitude. valuesjor ',"yvhichjthe transmittedsignal is capable into alxednumbermof constitiuent .ranges 'whichtogether encompass the total range. "Each of these smalleror constituentam-` plitudeV ranges may thenk betreated' as ifnit were a singleamplitudeJI instead'fffairange and is repwhich is of the valuerepresenting the presence of a particular prtion of the total amplituderange is weighted iii accordance with the amplif tude. portion which itrepresents and the Weighted pulses are combined to obtain a single pulsewhich is proportional to the sample amplitude. Successive reconstructedsample amplitude' pulses `are thencombined to reconstruct the originalsignal wave. v Various devices have been provided for per-I ionmingthe.decodingprocess One efficient de? vice iorlu'sef'when the code pulsesare sent in timesequence particularly with the pulserepre vsenti'ng thesmallest amplitude portion transl mitted virst'isthat disclosed inFrench Patent No. 960,862, issued November 7, 1949. The decoding systemdisclosed inthe patent referred to above is arranged to decode codegroups in which on pulses'representtthe.presence inthe sampledamplitu'de,` of fa fixed portion of the total amplitude Irange. '4The-,pulses of the received code group are applied to' a storage anddischarge circuit, each onfi'wpulstej being employed to produce apredetermined change in the amount of charge stored infthestoragecircuit. vThetime constant of the, circuit i'sfso' chosen that in theinterval between successive code element pulses the ,total amountoflYchange in the storage circuit changes yatane,xponental rate toavaluewhich islelated to' the Valueatthe beginning of the interpulseinterval bya; factor of 2. .At the completion of aco'defgroup the totalcharge then presentin the storage circuit is sampled. The sample am?plitudehso obtained represents the sum of the changes in'chargfeffproduced- .by the several code pulses. 'Ifffor example, a codegroup of '7 code 4elexrnzntsis employedvand-the contribution .due

. to"a.n" on pulser received in the unal code eleresented by an'individual'A one fofv the permutaL tions of the code. In-fthe use ofthis method of code transmission the instantaneous amplitude ascertainedby the sampling operation is represented by the respective permutationindicative of the amplitude range, or step, which most nearlyapproximates the amplitude of .the measured sample. nearest to thatamplitudel represented by the ninth step of the'total amplitude rangeythe permutation code correspondingto range 9 is transmitted.

If, for: example, the sample amplitude is It vrill be observed thateach'lcode element in one of its values represents the presence in thesampled `amplitude of a particular` xed portion of the total amplituderange, While in the other value it represents the ,absence of., thatsame .upon the mentpositionis taken at the time of sampling tocrinale/the total amplitude range, the contribu-f tion s due ta receivedcodepulses of the on valueQi l .tions mayy represent respectively 1/4,1/8, lg, gg, germand/ggtotal,amplitude range. Thus, anyamplitude'lvvithin the total amplitude range facceptableby Vthe systemmay be represented to Withinjygg the, total range. V

The "ecient operation of the decoding arrangement'ldescribed"abovedepends upon the ac'- uratespacingof the code element pulses randprec/isctimingof*` the sampling opera# tion. .l s (will be. understoodfrom a considera.- tio'ri orthe'fact that' variations of the' positionsO f 'the'A ode element pulses away from theiras'fl signedp'o'sitionsresult in the changing oi char-ge injthestorage circuit at times otherthanthose at the r.Specied exponential changesin charge from.. thepreceding vinterpulse intervals havefoccurredr. Similarly, since thetotal stored decoding process each'of the code element pulses soMcharee,variesceritiru1oi1s1y .after the receipt,` pf

tsucc,csfsvely earlier 60de element posithe last pulse of the codegroup, inaccuracies in the timing of the sampling operation with respectto the completion of the code group result in errors as to the sampleamplitude repre-- sented by the code group. f

Variations in the interpulse spacing or jitter of the pulses may occurin radio systems such as those often employed between the sending ,and

receiving stations of a pulse code modulation system. The undesirableeilect of such variations which are usually caused by noise potentialsmay be eliminated to some extent by precise clipping and limting of thereceived pulses. This solution of the problem involves the provision ofadditional circuits in the synchronizing equipment at the receivingstation. The accurate timing of the sampling operation in relation tothe ,times o f the received pulses places severe requirements on thesynchronizing systems, necessitating complex circuit arrangements.

Accordingly it is an object of the present invention to eliminate theneed for precision timing circuits in pulse code modulatingsystemsemploying decoders of the type described above. It is an additionalobject of the invention to improve the performance of such decodingcircuits by reducing errors caused by variations in the timing of codeelement pulses and sampling operations.

In accordance with the present invention the exponential characteristicrelating time and change of charge on the decoder storage circuit`during the interpulse intervals is modified to provide inflectionpoints during which the slope of the characteristic is essentially zero.Such infiation points are caused to occur at intervals `equal to theinterval between the code element pulses land the sampling operation isperformed at times corresponding to such inflection points.

The above and other features ofthe invention may be understood from a.consideration of the following detailed specification taken inconnection with the drawings in which:

Fig. 1 is a schematic diagram partially in block form of a decodingcircuit in accordance with the invention; y

Fig. 2 is a schematic diagrampartially in block form showing amodification o f the arrangement of Fig. 1;

Fig. 3 is a graph showing the relationships l between certain quantitiesin the decoding vcircuit of Fig. 1;

Fig. 4 is a schematic diagram of the shown in Fig. 1; and

Fig. 5 is a graph illustrating operation of the decoder of theinvention.

In an illustrative embodiment o f the invention, the storage circuit ofthe decoder referred to above is modied through the addition to the RCstorage circuit of a damped antiresonantcircuit the frequency of whichis related to the code element pulse repetition frequency. Thus a dampedsinusoidal component may be added to the exponential characteristic` ofthe RC circuit.

Referring noW to Fig. 1 the RC portion of thc storage circuit comprisesa capacitor C1 and a resistor R1 connected in parallel. A dampedantiresonant or oscillatory circuit is connected in sesies with theexponential discharge circuit, the antiresonant circuit comprising acapacitor C2, a resistor R2 and an inductor L2 connected `in'parallel.This series combination is connected to the output of a constant currentgenerator which applies a predetermined amountr of charge to the storagecircuit in response to each codeelement pulse of the value representingthe presence of a portion of amplitude of the signal wave. A samplingcircuit is also connected across the storage circuit and is enabled atan appropriate time following the completion of a code group to providean output signal representative of the total amount of charge present inthe storage circuit at the time of sampling.

Considering the effect of a single code pulse upon the storage circuitit will be seen that in response to such a pulse both sections of thestorage circuit are excited simultaneously. In the section comprisingcapacitor C1 and resistor R1 a iixed amount of charge is added to thecircuit after which the potential thereacross decays exponentiallysubstantially as shown in curve I0 of Fig. 3. At the same time thedamped antiresonant circuit comprising capacitor C2, resistor R2 andinductor L2 is excited and a damped sine wave appears thereacross. Thisdamped sine wave is shown in curve I2 of Fig. 3.

By appropriate adjustment of the parameters of thestorage circuit, thecharacteristic curve Ill Aof the RC circuit and the lcurve l 2 of thedamped antiresonant circuitymay be so proportioned that their sum takesthe form shown in curve I4 of Fig. 3. In this curve, which representsthe characteristic of the decoding circuit of Fig. y1, inflection pointsof substantially zero slope occur at intervals which may be made equalto the interpulse intervals of the codegroups. It will be recognizedthat the effects of timing errors may be greatly reduced if thesamplingoperation is performed at one of the zero slope portions of thecharacteristic. Since these portions of the characteristic are of iiniteduration, the time of sampling may vary over a considerable rangeWithout causing errors in the value of the sample amplitude. Assumingthat the timing of the code element pulses is correct, the time ofsampling may vary over a period equal to the entire duration of theinflection in the characteristic. Similarly, if the sampling time iscorrect, the cumulative error in the timing of the code element pulsesmay vary over the same range. If both the sampling and code elementpulse times are varied, the sum of the timing errors may approachtheduration of the inflections of the characteristic without producing`decoding errors.

The circuit constants required for the production of the desiredcharacteristic may be determined experimentally or by mathematicalanalysis. Thus the characteristic due to the RC circuitmay be expressedby:

E 1 .V6-1 t and that due to .the antiresonant circuit by:

E2=Ac"2 sin at In the mathematical analysis, the expression for the sumof these two components is twice differentiated, and a solution isfoundsuch that both the first and second derivates are Zero.

It may be shown that in order to obtain the desired characteristic asshown in curve I4 of Fig. 3 the frequency f,= 1 21m/.MC2

must be made equal to the pulse repetition frequency of the code elementpulses. Similarly it may be shown that the decrement antiresonantcircuit 4f theRC storflge` .Circuitf must equal that-of the As indicatedabove, appropriate Avaluesforl the parameters of the storage circuitsmaybeobtained either experimentally or mathematically. It will be found,however,y that theoretically defrived valuesmust be modified toaccount'for the internal impedance of thev constant current source"employed to chargethe storage circuit and the impedance of Iotherapparatus associated with the storage circuit. A 'suitable arrangementAfor producing suchV compensation is shown in .`ig.j2- in vwhich the"parallel combination fof a resistor R5 and a capacitor Ce is-'connectedin parallel with the storage circuit across the con- Istant currentsource. Since the components of the storage `circuit must then havedifferent values they are relabelled R3, C3 for the exponential' circuitand R4. C4, L4 for the anti-- resonant circuit.' Through the use of wellknown impedance transformationsv the following. relationships betweenthe valuesof C1', Cz, R1, and L2 eifrig; 1v, amica, C4, R4, R5 auch ofFiga maybe" obtained in terms of the qua'ntity'Cs which represents thesum of the internalimpe'dance of the source and yassociated apparatus "iIt may be shown experimentally that R5 can be eliminatedwithout'affecting the operation fof th system providing suitablechanges'are .made inlthe values of the remaining vcircuit Vcompo-'-nents. lHoweventhe inclusion of R5 greatly siml'plifles-"tlne impedancetransformations necessary for the specication of .the other components.The vactual circuit'values for all of the components-of a practicalembodiment of the invention 'may be computed from the` above equationswhich` expressthem Ain terms of the elements of the theoretical circuitof Fig. 1. y i f illustrativevembodiment of an improved decoder inrvaccordance with the invention is shown in schematic form in Fig. 4. Inthis embodiment the Vstorage lcircuit'cornprising the series combinationof resistor l6 and capacitor Ill2 in parallel and-inductor, resistor 22and capacitor. vin vparallel connected in: the. anode .circuit of a,pentode-type vacuum tube 26 which isv arranged to serve as a constantcurrent generator. Accordingly, a fixed positive potential obtained frombattery 2S is applied to thescreen grid of` pentode 26 while the pulsecode modulation pulses are applied through a coupling capacitor 29 tothe control grid, the usual grid return being furnished by resis-tor 30.The cathode of pentode 26 is connected to ground through n an unbypassedlresistor 32 while the anode is connectedA through the storage circuitto the positive terminal of an anode supply battery 34.

'g yIt is well known that a pentode-type tube with l ceived, a pulse ofconstant current vis applied to the storage circuit from anode battery34. This excites the two sections of the'storage circuit which thendischarges as shown in curve I4 of Fig. 3 Each successive on codeelement pulse causes the further excitation of the storage circuit. Thecumulative effect may be represented by the diagram of Fig. 5. Here asan example there is assumed a code group of seven elements in which thefourth and sixth code element pulses are on pulses and the remainder areoff pulses. The off pulsesare not effective to produce any change in thecharge inthe storagecrcuit. Upon receipt of lthe `fourth pulse which isan on pulse the'storage circuit ig charged negatively to a.predetermined value. The charge begins to decay at once and intheabsence of additional pulses would reach a value at the sampling Atime(indicated by the arrows in Fig. 5.) equal to the total range. Beforethe charge decays to this value, however, an additionall charge of thesame predetermined value is added to the circuit in response to thesixth pulse which is also an on pulse.- The total charge is increasedbut begins again -toI decay as shown in Fig. 5. In the absence ofthecharge due to the fourth pulse, that due to the sixth ,pulse `wouldcause the charge to vary as shown by the dashed line in Fig. 5, reachinga value at the time of sampling representing 1/4 the total amplituderange. The components due to the fourth and sixth pulses add, however,giving a value at the time of sampling representing 11g-l- 1@ or thetotal range of the system. After the charge in the storage circuit hasdecayed to zero, the circuit is ready to decode another code group.Since the code groups are usually sent in succession with little or nointerval between groups, it becomes necessary, in the application ofthis type of decoder, either to provide means for rapidly dischargingthe storage circuit immediately after the sampling yoperation or toprovide one or more additional decoders which may be utilized in turn todecode successive code groups.

AThe* voltage across the storage `circuit is applied to the control gridof a triode-type tube 4U which connected as a cathode followerarnplirerand the output of this tube, appearing across cathode. resistor42 is yapplied ,to .asam- 'pliiig circuit. This cathode follower stageis employed to avoid loading down the storage circuit by the inputimpedance of the sampling equipment.

The sampling circuit may be of any suitable type such that a` quantity'proportional tothe voltage across the storage circuit may be produced atpredetermined instants. As shown herein the sampler comprises triodes 44and 46 connected back to back between the cathode of cathode follower 40and a storage capacitor 48. Normally, the new or current throughtrlode's 44 and' Iii;l is cut off but these tubes may be enabled atpredetermined times by the application of sampling pulses-whichareapplied through transformer 50. This transformer has dual secondarywindings which are included in the grid circuits of triode tubes at andEilrespectively. During sampling pulses triodes i4 and 46 are madeconductive and the voltage across storage capacitor 48 is made equal tothat of the cathode of cati-'iode follower dil. Between sampling pulsestriodes 44 andi @t are cut off andi the voltage across storage capacitor48 is held at the value which it had at the conclusion of the precedingsampling pulse. This type of circuit is sometimes referred toas aclamping circuit'. Thus it will be understood that an output voltage maybe obtained` from storage capacitor is which is a measure of the voltageacross the decoding storage circuit at the time of sampling and is thusa, measure of the sample amplitude applied to the decoding circuitduring the preceding interval.

In the above embodiment a single sinusoidal componenti is added' to thedischarge characteristic of the RC storage circuit to obtain anapproximation of a stepped discharged wave. This modiiication is capableof effecting large reductions in the errors due to imperfect timing.Additional imrovements in performance may be obtained by adding otherharmonically related sinusoidal components to the discharge charac'-teristic, the amplitudes and decrements of these components beingrelated to those of the exponentialwave. Such components may be added byprovision of additional damped antire'sona'nt circuits, similar to thatemployed in thed'escribed embodiment, in series with the RC dischargecircuit.

Freedom from the effects of pulse jitter may also be obtained from theuse of harmonically related sinusoidal components without the RCexponential component by sampling the resulting decoder voltagewaveforms in the center of the resulting fiat regions. However, the useof the sinusoidal voltage component with the RC3` exiponential wavegives a greater reduction in the decoder errors with a simpler circuitconfiguration.

Whatl is claimed is:

1. In a system for decoding code groups" of bivalued pulses normallyoccurring at equal intervals, each pulse in one value representingv aportion ofthe amplitude of a signal wave, a storage circuit having anexponential dischargeA characteristic such that during each of theinterpulse intervals the energy is reduced to a predetermined fractionof that present at the beginning of the interpulse interval, means forstoring a predetermined amount of energy in said' storage circuit inresponse to each pulse of` said one value, means in series with saidstorage circuit and responsive to each pulse of said one valueforfgeneratingaV damped sine wave,l the period ofvvhich is related tothe interpul'se intervals of said bi-valued pulses and the amplitude anddecrement of which are related to said discharge characteristic, andmeans for sampling the sum of the energies present in said storage andsaid sine wave` generating circuits at predetermined times.

2. In a system for decoding code groups of bivalued pulses'L normallyoccurring at equal intervals, each pulse in' one value representing aVpor'- tion of the amplitude of a signal wave, a storage circuit havingan exponential discharge characteristic such that during each of the4interpulse" intervals the energy is reduced to a predetermined fraction'of that present at the beglnning of the interval, means for storing apredetermined amount of energy in said storage crcuit in response toeach pulse of said one value, a damped oscillatory circuit connected inseries with said storage circuit for excitation by each pulse'V of saidone' value, the frequency of said oscillatory circuit being equal to therepetition rate of said loi-valued pulses and the amplitude anddecrement of said circuit being related to the corresponding quantitiesof said storage circuit, and means for sampling the sum ofthe energiesstored in said storage and oscillatory circuits at predeterminedinstants.

3. In a system for decoding code groups of bivalued pulses normallyoccurring at equalintervals, each pulse in one value representing aportion of the amplitude of a signal wave, a storage circuit having anexponential discharge characteristic such that during each interpulseinterval the energy is reduced to a predetermined fraction of thatpresent at the beginning of the interval, means for storing apredetermined amount of energy in said storage circuit in response toeach pulse of said one value, means in series with said storage circuitfor generating a damped sine wave in response to each pulse of said onevalue, the frequency of the sine wave being equal to the repetitionfrequency of said bi-valued pulses, the decrement of said damped sinewave being equal to that of said exponential discharge circuit and theamplitude of said sine wave being equal to 11 per cent of that of theexponential wave of sa'id storage circuit and means for sampling thetotal amount of energy stored in said storage and sine Wave generatingcircuits.

4. In a system for decoding code groups of bivalued pulses normallyoccurring at equal intervals, each pulse in one value representing aportion of the amplitude of a signal wave, a storage circuit having anexponential discharge characteristic such that during each interpulseinterval' the energy is reduced to apredetermined fraction of thatpresent at the beginning of the interval, means for storing apredetermined amount of energy in said storage circuit in response toeach pulseA of said one value, a damped oscillatory `circuit connectedin series with said storage circuit for excitationl by each pulse ofsaid onevaina-theconstants of said oscillatory circuit being so chosenthat its period is' equal to the inner pulse intervals',I the amplitudeof the sine wave is equal to 11 per cent of the exponential wave of saidstorage circuit and the decrement of the sinewave is equal to that ofthe exponential wave; and means for sampling the combined energy storedin; said storage'and oscillatory circuits.

5i. In ay system for decoding code groups of bivaluedkpulsesnormallyoccurring at equal intervals, each pulse in one value representing aportion of the amplitude of a signal Wave, an RC storage circuit havinga discharge-characteristic such that during each interpulse interval theenergy is reduced to a predetermined fraction of that present at thebeginning of the interval, means for storing a predetermined amount ofenergy in said ycircuit in response to each pulse of said one value, adamped antiresonant circuit connected in series with said storagecircuit for excitation by said pulses of said one value, the period ofthe Wave generated :by said antireso" nant circuit being related to theinterpulse intervals and the amplitude and decrement of saidantiresonant circuit Wave being related to the corresponding quantitiesof the storage circuit output and means for sampling the total amount ofenergy stored in said storage and antiresonant circuits at predeterminedinstants.

6. In a system for decoding code groups of multivalued pulses normallyoccurring at equal intervals and each pulse in one value representing aportion of the amplitude of a signal wave, a rst energy storage devicehaving a characteristic such that during each interpulse interval theenergy stored in the device is changed by a fixed fraction of thatpresent at the beginning of the interval, a second energy storage devicehaving a periodic characteristic with a period equal to said interpulseintervals, means for storing predetermined amounts of energy in each ofsaid devices in response to each pulse of said one value, and meanseffective at predetermined times for sampling the sum of the energiesstored in said devices.

7. In a system for decoding code groups of multivalued pulses normallyoccurring at equal intervals and each pulse in one value representing aportion of the amplitude of a signal wave, means responsive to eachpulse of said one Value to generate an exponential wave which, duringeach pulse interval, decays to one-half the amplitude existing at thebeginning of the interval, means also responsive to each pulse of saidone value to generate a second Wave for combination with saidexponential wave, said second Wave having portions which occur atintervals equal to said interpulse intervals and have a slope Which issupplementary to that of said exponential Wave I at these intervals, andmeans for sampling the amplitude of the sum of said waves atpredetermined times.

8. In a system for decoding code groups of multivalued pulses normallyoccurring at equal intervals and each pulse in one value representing aportion of the amplitude of a signal Wave, means responsive to eachpulse of said one value to generate an exponential wave which, duringeach pulse interval, decays to one-half the amplitude existing at thebeginning of the interval, means also responsive to each pulse of saidone value to generate a second wave for a combination with saidexponential Wave, said second Wave having portions which occur atintervals equal to the interpulse intervals and have a slope which isequal to that of said exponential Wave but of opposite sign, and meansfor sampling the amplitude of the sum of said Waves at times when theslope of the combined waves is substantially zero.

9. In a system for decoding code groups of multivalued pulses normallyoccurring at equal intervals each pulse in one value representing aportion of the amplitude of a signal Wave, means responsive to eachpulse of said one value to produce an exponential Wave which, duringeach pulse interval, decays to a value one-half that present at thebeginning of the interval, means also responsive to each pulse of saidone value to produce a periodic wave having a period equal to saidinterpulse interval and having a slope at such intervals such that thecombination of the two waves at these times has rst and secondderivatives which are simultaneously substantially zero, and means forsampling the sum of the two waves at said times.

10. In a system for decoding code groups of multivalued pulses normallyoccurring at equal intervals each pulse in one value representing aportion of the amplitude of a signal Wave, a rst energy storage devicehaving a discharge characteristic such that during each interpulseinterval the energy stored in the device is reduced by a fixed fractionof that present at the beginning of the interval, a second energystorage device having a discharge characteristic which variesperiodically with a period equal to said interpulse intervals, a sourceof current impulses arranged to apply predetermined amounts of energy toeach of said devices in response to each pulse of said one value, meansfor compensating the internal impedance of said source of currentimpulses, and means for sampling the sum of the energies in said devicesat predetermined times.

11. In a system for decoding code groups of multivalued pulses normallyoccurring at equal intervals each pulse in one value representing aportion of the amplitude of a signal wave, a rst energy storage devicehaving a discharge characteristic such that during each interpulseinterval the energy stored in the device is reduced by a xed fraction ofthat present at the beginning of the interval, a second energy storagedevice having a discharge characteristic which varies periodically witha period equal to said interpulse intervals, a source of currentimpulses arranged to apply predetermined amounts of energy to each ofsaid devices in response to each pulse of said one value, and an RC'network connected across the inputs of said devices for compensatingthe internal impedance of said source.

ALOIS J. RACK.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS- Number Name Date 1,733,614 Marrison Oct, 29, 19292,277,000 Bingley Mar. 17, 1942 2,292,835 Hepp Aug. 11, 1942 2,419,772Gottier Apr. 29, 1947 2,438,907 Frankel et al. Apr. 6, 1948

